1. Field of the Invention
The present invention relates to a lithography method, and more particularly, to a lithography method utilizing a selective chromeless phase shift mask (PSM) to form patterns having a superior resolution and contrast in a photoresist layer.
2. Description of the Prior Art
In integrated circuit manufacturing processes, a lithographic process has become a mandatory technique. In a lithographic process, a designed pattern, such as a circuit pattern, a doping pattern, a contact hole pattern, or a trench pattern, is created on one or several photo masks, then the pattern on the photo mask is transferred by light exposure, with a stepper and scanner, into a photoresist layer on a semiconductor wafer. Only by using a lithographic process can a wafer producer precisely and clearly transfer a complicated circuit pattern onto a semiconductor wafer.
It is an important issue for solving resolution of the lithographic process due to the reducing device sizes of the semiconductor industry. Theoretically, using short wavelengths of light to expose a photoresist layer will improve the resolution right away. Short wavelengths of light are desirable as the shorter the wavelength, the higher the possible resolution of the pattern. This method, though it seems simple, is not feasible. First, light sources for providing short wavelengths of light are not accessible. Secondly, the damage of equipment is very considerable when short wavelengths of light is used to expose a photoresist layer, leading to a shortened equipment lifetime. The cost is thus raised, which makes products not competitive. Due to the conflicts between theory and practice used in manufacturing, the manufacturers are all devoted to various researches so as to overcome this problem.
Please refer to FIG. 1, FIG. 1 is a schematic diagram illustrating a prior art method for improving a resolution of contact hole patterns 24 by utilizing an attenuated phase shift mask. As shown in FIG. 1, the attenuated phase shift mask 10 is formed from a quartz substrate. A plurality of contact hole features 12 and a not completely transparent region 14 enclosing each of the contact hole features 12 are included on attenuated phase shift mask 10. Actually, the not completely transparent region 14 of the attenuated phase shift mask 10 is coated with a single-layered embedded layer 16. The embedded layer 16 is not only an absorption layer, but also is a phase shift layer. Since the transmittance and the phase shift angle required by the process can be achieved by utilizing the single-layered embedded layer 16, the embedded layer 16 is also called an absorptive shifter. In addition, the attenuated phase shift mask is also called a half-tone mask.
When a specific wavelength of light (correlating to the embedded layer) is utilized to perform the exposure process, light will pass through each of the contact hole features 12 successfully to reach a photoresist layer 22 on a wafer 18. Because the embedded layer 16 has a specific transmittance (usually between 2-15%), portions of the light passing through the embedded layer 16 will have a phase shift of 180 degrees relative to the original light, and will result in destructive interference with the light passing through the contact hole features 12. After develop and bake processes are performed, the contact hole patterns 24 defined by the residual photoresist layer and corresponding to the contact hole features 12 will be formed in the photoresist layer 22. Thanks to the interference effect caused by light passing through the embedded layer 16, the resolution of the contact hole patterns 24 is improved.
Please refer to FIG. 2, FIG. 2 is a schematic diagram illustrating a stepper 30 utilizing an on-axis illumination method. As shown in FIG. 2, light beams originating from a light source 32 pass through condenser lens 34 first, then evenly illuminate a mask 36 having contact hole patterns (not shown) on it. Diffraction effects occur because the mask 36 hinders incident light. The even incident light is thus separated into diffraction light of different orders. The diffraction light of different orders is thereafter incident upon projection lens 38 to allow the projection lens 38 to collect the diffraction light of different orders and to focus them on a wafer 42. However, the smaller the critical dimension (CD) is, the larger the diffraction angle of the incident light is with the same exposure light source, theoretically. When the critical dimension of the contact hole patterns (not shown) is very small, the diffraction angle of the incident light becomes very large. Under the circumstances, the projection lens 38 can only collect the zero order (0 order) light from perpendicularly incident light beams due to the large diffraction angles, leading to images having poor resolution on the wafer 42, or even leading to unexposed images on the wafer 42.
In order to resolve this problem, an attenuated phase shift mask is usually used in conjunction with a stepper utilizing an off-axis illumination (OAI) method. Please refer to FIG. 3, FIG. 3 is a schematic diagram illustrating a stepper 50 utilizing an off-axis illumination (OAI) method. As shown in FIG. 3, the light beams illuminate a mask 54 having contact hole patterns (not shown) on it with a specific incident angle when they are transmitted through the off-axis aperture 52. The mask 54 will then diffract the light beams passing through it and allow zero and first order diffracted light interfere on a surface of a wafer 56 and contribute to the image formation. However, the incident angle needs to be adjusted according to the pattern spatial frequencies, and the exposure time needs to be increased in order to compensate for the loss of illumination due to tilting. In summary, the illumination is tilted symmetrically to an angle to allow the un-diffracted light (the zero order light) and the first-order diffracted light (the +first order light) to pass through the mask 54 and interfere on wafer 56 in this technique. Since both the zero order light and the +first order light are collected by projection lens 58 and are focused on the wafer 56, images are formed on the wafer 56 successfully. As a result, the off-axis illumination method enhances the resolution and depth of focus (DOF) of the lithographic process to produce contact hole patterns (not shown) in a photoresist layer (not shown) on the wafer 56.
However, the above-mentioned method has limitations, both in critical dimension and critical dimension uniformity. In addition, when utilizing the attenuated phase shift mask to improve the resolution of the contact hole patterns or other dense patterns, the light intensity of the side-lobe is too high to produce extra patterns if the transmittance of the embedded layer is too high. If the transmittance of the embedded layer is too low, the destructive interference of light caused by the phase shift is not enough. The side-lobe phenomenon at the edge of contact hole patterns or other dense patterns cannot be eliminated, leading to the unsatisfied pattern edge resolution.
Furthermore, the method involving adjusting the off-axis illumination method related parameters has been adapted in order to achieve a better resolution and make a compromise between resolution and depth of focus. However, satisfied results are not obtained. At the same time, man-power and time are wasted and equipment is damaged. Therefore, it is very important to develop a lithography method to improve the resolution and contrast of the contact hole patterns and other dense patterns effectively. This method is able to be applied to small-sized patterns, and should not damage equipment when using the current equipment.